The invention relates to a display apparatus and display method thereof. A display apparatus, such as a liquid crystal display (LCD), a plasma display panel (PDP), and a digital micromirror display (DMD), is controlled to display luminance gradations (gray level) by a time-sharing drive method for displaying an image by selectively illuminating pixels arranged in a matrix-form.
A prior art example of a plasma display apparatus will be described using the example of a matrix display device. A plasma display device is roughly classified into AC and DC types.
FIG. 1 is a block diagram illustrating the outline of a DC-type plasma display device. A plasma display device 10 is constituted by a display panel 11, a plurality of address electrodes 15, a plurality of scanning electrodes 16, an address pulse generator 12 for driving the address electrodes 15, a scanning and sustaining pulse generator 13 for driving the scanning electrodes 16, and a signal processing circuit 14 for controlling the generators 12, 13. The display panel 11 is provided with two spaced glass plates, the address electrodes 15, the scanning electrodes 16, and a partition for partitioning the space between the two glass plates. A pixel is constituted by a discharge cell which has space partitioned by a partition between the two glass plates. For example, a rare gas, such as He--Xe (helium-xenon) and Ne--Xe (neon-xenon), is enclosed in each discharge cell and when a voltage is applied to a selected address electrode 15 and a selected scanning electrode 16, a discharge occurs and ultraviolet rays are generated. A color display can be produced by coating every discharge cell with a red phosphor, a green phosphor and a blue phosphor and by selecting a phosphor or phosphors according to an image signal.
FIG. 2 illustrates the drive waveform of a DC-type plasma display. In FIG. 2, numeral 30 denotes the drive waveform of the DC type plasma display. The electrodes 15 and 16 are driven in a line sequential manner. An address pulse 31 having a voltage of VA is supplied depending on a picture signal, to an address electrode 15 which corresponds to the discharge cell in the Nth row. In the meantime, a scanning pulse 32 having a voltage of VS is supplied to the scanning electrode 16 in order from the first line. The address voltage VA and the scanning voltage VS are simultaneously supplied to a cell. When a voltage between the electrodes 15 and 16 exceeds the discharge starting voltage, the cell is discharged. This discharge is an address discharge. In a fixed period after discharge, the discharge is sustained by a lower voltage than discharge starting voltage because a charged particle is left in the discharged cell. Therefore, in a cell in which an address discharge occurs, the discharge is continued by a sustaining pulse 33 having a voltage of VS2 supplied next to a scanning pulse 32. Such a driving method is called a memory drive method.
Next, the method for displaying gradations of luminance will be described using a time sharing drive method utilizing the above memory drive method (or a sub-field system). The sub-field system is a method for realizing multiple gradations by dividing one field into plural sub-fields weighted according to the difference in the luminance or brightness and selecting an arbitrary sub-field every pixel according to the amplitude of a signal. The word "field" used in this specification means a vertical scanning period and sometimes is called a "frame", and a "sub-field" is called a "sub-frame".
FIG. 3 illustrates an example of a drive sequence of a prior plasma display apparatus of DC the type. A drive sequence 40 utilizing the time sharing drive method shown in FIG. 3 is an example in which an image is displayed in sixteen gradations by four sub-fields SF1 to SF4. A scanning period 41 indicates a period for selecting a light emitting cell in a first sub-field and a sustaining period 42 indicates a period in which the selected cell emits light. Each sustaining period of the sub-fields SF1 to SF4 is weighted so that the luminance ratio of the sub-fields is 8:4:2:1, and if the luminance of these sub-fields is optionally selected according to the level of an image signal, a display in sixteen gradations equivalent to the fourth power of two is enabled. If the number of gradations is to be increased, the number of sub-fields has only to be increased, and, for example, if the number of sub-fields is eight, and the luminance ratio during the sustaining period is to be selected 128:64:32:16:8:4:2:1, a display in two hundred and fifty-six gradations is enabled. The luminance level of each sub-field is controlled by the number of pulses supplied during the sustaining period. This type of plasma display apparatus and the driving method are disclosed, for example, in SID94DIGEST (page 723-726).
FIG. 4 is a block diagram illustrating the outline of an AC-type plasma display device. The plasma display device 20 is constituted by a display panel 21, a plurality of address electrodes 26, a plurality of scanning electrodes 27, a plurality of sustaining electrodes 28, an address pulse generator 22 for driving the address electrodes 26, a scanning and sustaining pulse generator 23 for driving the scanning electrodes 27, a sustaining pulse generator 25 for driving the sustaining electrodes 28, and a signal processing circuit 24 for controlling the generators 22, 23, 25. The display panel 21 is provided with two spaced glass plates, the address electrodes 26, the scanning electrodes 27, the sustaining electrodes 28, and a partition for partitioning the space between the glass plates. A pixel is constituted by a discharge cell which has space partitioned by the partition between the two glass plates. The AC-type plasma display is different from the DC-type display in that an electrode is covered with a dielectric. Rare gas such as He--Xe and Ne--Xe is enclosed in each discharge cell, and if a voltage is applied between the address electrode 26 and the scanning electrode 27, a discharge occurs and ultraviolet rays are generated. A color display can be produced by coating every discharge cell with a red, a green and a blue phosphor and by selecting it according to an image signal.
FIG. 5 illustrates the drive waveform of an AC-type plasma display. In FIG. 5, numeral 50 denotes the drive waveform of the AC-type plasma display. The electrodes 26 and 27 are driven in line sequence and an address pulse 51 having a voltage VA is supplied, depending on an image signal, to an address electrode 26 corresponding to a discharge cell in the Nth row. In the meantime, a scanning pulse 52 having a voltage VS is supplied in order from the first line to a scanning electrode 27. The address voltage VA and the scanning voltage VS are simultaneously supplied to a cell. When the voltage between the address electrode 26 and the scanning electrode 27 exceeds the discharge starting voltage, the cell is discharged. Assuming that this discharge is an address discharge, in a cell in which discharge occurs, a charge is stored on a dielectric covering an electrode (hereinafter called a wall charge), and in a fixed period after it, the discharge can be sustained by a lower voltage than the discharge starting voltage. In the example shown in FIG. 5, the scanning electrode 27 also functions as a sustaining electrode and a sustaining discharge is caused by alternately supplying a sustaining pulse 53 to the scanning electrode 27 and the sustaining electrode 28. At this time, the direction of the discharge by the scanning electrode 27 and the sustaining electrode 28 is alternately changed. Therefore, the plasma display is referred to an AC type display. Such a drive method is called a memory driving method as in the case of the DC type display, and the AC-type plasma display can be driven in a drive sequence 40 as shown in FIG. 3 similar to the DC-type display. However, since the duration of the memory effect caused by a wall charge is longer, compared with that of the memory effect caused by a DC-type charged particle, another drive sequence is also proposed.
A drive sequence 60 by a time sharing drive method shown in FIG. 6 is an example of a case in which an image is displayed in sixteen gradations by four sub-fields SF1 to SF4. A scanning period 61 is a period for selecting a light emitting cell in a first sub-field SF1, and a sustaining period 62 is a period in which the selected cell emits light. Each sustaining period of the sub-fields SF1 to SF4 is weighted so as to have a luminous ratio of 8:4:2:1, and if the luminance of these sub-fields is arbitrarily selected according to the level of an image signal, a display in sixteen gradations equivalent to the fourth power of two is enabled.
As described above, the principle of the time sharing drive method is the same as that of the above DC type shown in FIG. 2, however, the time sharing drive method of the AC type is characterized in that the scanning period 61 and the sustaining period 62 are completely separated and the sustaining pulse 53 common to the whole screen is supplied to the sustaining period 62. This type of apparatus is disclosed on pages 7 to 11 in SHINGAKUGIHOU (Communications Institute Technical Report), EID 92-86 issued in January, 1993, for example.
In case a dynamic image taken by a camera is displayed by using the time sharing drive method, it has been reported that a disturbance, which is referred to as dynamic false contours or quantum noise, is brought about by the time sharing drive sequence. The disturbance or the noise is caused by a change in the light emitting interval which is varied by the display gradations and by the shift of one's eye followed by the dynamic image. To solve this problem, a high-ranking bit, which has large luminous weight, is divided into two and is emitted in different periods. When the high ranking 4 bit in the sub-fields having a luminous ratio of 8:4:2:1 is assigned to a digital image signal, for example, the highest ranking bit is divided into two and the number of the sub-fields is increased from 4 to 5. Then, the luminous ratio of the sub-fields becomes 4:4:2:1:4, and for the highest ranking bit, the first sub-field and the last sub-field are assigned. This is one of the ways to decrease or to suppress the dynamic false contours. Various proposals for a method for dividing the sub-field and the order for emitting the divided sub-field have been made. This kind of method has been described in, for example, SDI DIGEST 96 (page 291-294).
Presently, there is a demand for a display device which is provided with a high resolution and multiple gradations to correspond to any media. Particularly, by the wide-spread use of the photo CD and MPEG software, a display apparatus for displaying a high resolution image taken by a camera is required. In a case where a display apparatus with a high resolution is used, plural windows are provided on the screen and a dynamic image is displayed on one of the windows. In the field of television receivers, a so-called wide television having an aspect ratio of 16:9 is the subject of increasing interest in the market. Therefore, a dynamic image having an aspect ratio of 16:9 is required for display on a display device having aspect ratio of 3:4.
In the above sub-field system according to the prior art, it is difficult to increase the number of the sub-fields because a longer period is needed to increase the number of the scanning lines. On the other hand, it is necessary to increase the number of the sub-fields in order to increase the number of gradations, or to reduce the dynamic false contours by dividing the higher ranking bit. Therefore, providing both an improvement in the resolution and an improvement of the picture quality is very difficult. In case a dynamic image is displayed on a window on the display panel equivalent to XGA (1024768 dot), and for a window which corresponds to the VGA system, the number of scanning lines of an XGA display are 1.6 times that of a VGA display. The time required for scanning the sub-fields of the XGA display is also 1.6 times that of the VGA display. Therefore, the sustaining period is shortened and sufficient brightness is not obtained, or the number of sub-fields is reduced and sufficient gradations are not obtained. In this case, the image on the XGA display is deteriorated and becomes an unnatural image in comparison with the image of the VGA display.